The use of screen-printable dielectric compositions in paste form to form printed capacitors for use with silver-bearing terminations is relatively new in the art of constructing electronic systems.
In general, such dielectric compositions contain a ferro-electric material, preferably BaTiO.sub.3, glass, organic vehicle and optionally certain inorganic materials such as ZrO.sub.2, Fe.sub.2 O.sub.3, for example as Curie point shifters. Such compositions which are available commercially can be printed over precious metal electrodes and then be cofired with a top electrode to form capacitor. The stability of these printed capacitors is affected adversely mainly in two ways: (1) by low thickness; and (2) by water absorption. The dielectric thickness must be greater than a certain minimum value, depending on the maximum dielectric constant (K). The major dielectric property affected by a thin dielectric with moderately high K (defined here as a K of about 50 or above) is the insulation resistance (IR). Under load, very thin dielectrics show a decrease in IR values during aging. Thicker dielectrics generally do not show such a degradation in properties. Though it is not known with certainty, it is likely that this degradation in insulation resistance is related to the diffusion of ionic species from anode to cathode.
In addition, reversible absorption of water leading to ionization and migration of ionizable oxides is also a major problem of screen-printed capacitors. In the past, this has generally been eliminated by encapsulating the whole capacitor with organic or inorganic materials such a epoxy resins, silicones, glasses and the like. However, encapsulation processing is very costly and could be avoided with the development of a dielectric system which does not absorb moisture and, if moisture is absorbed, the materials in the system do not ionize and/or migrate species from the anode to cathode.